[pep-8001] Define tactical voting

pep-8001.rst

@@ -144,7 +144,35 @@ important decision like governance, we owe it to ourselves and the wider
 Python community to be transparent about how the choice was made.
 This removes ambiguity around *who* voted and *how*, as well as allows
 people to confirm whether any "tactical voting" occurred (which instant
-run-off ranked voting is criticized for).
+run-off ranked voting is criticized for; see below).
+
+Are there any deficiencies of instant run-off ranked voting?
+------------------------------------------------------------
+
+There is no perfect voting method.  It has been shown by the
+`Gibbard-Satterthwaite theorem
+<https://en.wikipedia.org/wiki/Gibbard%E2%80%93Satterthwaite_theorem>`_
+that any single-winner ranked voting method which is not dictatorial
+must be susceptible to so-called "tactical voting".
+
+Tactical voting occurs when a voter supports a candidate against their
+*sincere preference* in order to prevent an outcome they find most
+undesirable.  There are `four major tactical voting strategies
+<https://en.wikipedia.org/wiki/Tactical_voting>`_
+(compromising, burying, push-over, and bullet voting).
+
+Instant run-off ranked voting is resistant to burying and bullet voting,
+while being somewhat vulnerable to compromising (less than the plurality
+method) and vulnerable to push-over voting.  Let's summarize those two:
+
+* compromising - the voter ranks a less desirable alternative higher
+  because they believe it has a higher chance of being elected; this is
+  sometimes called "casting a useful vote");
+
+* push-over - if the voter is relatively sure their preferred candidate
+  will survive the first counting round, they may rank "the weakest"
+  alternative higher in the hope of that weak alternative being easily
+  beatable in a subsequent round.
 
 
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